1. Technical Field
This invention relates to a solder-precoated conductor circuit substrate and a method of producing the same, and more particularly to a solder-precoated conductor circuit substrate capable of mounting components in a high packaging density and having an excellent productivity and a method of producing the same.
2. Background Art
Recently, conductor circuit substrates for mounting electronic components such as IC, LSI and the like are demanded to have a fine pattern with the miniaturization and high densification of these electronic components. For this purpose, more precision is required even in the connection of surface mounting components to the conductor circuit substrates.
As a technique of connecting the surface mounting component to the conductor circuit substrate, there is widely used a soldering method, particularly a reflow soldering method with a high productivity. This method is a technique in which a solder paste is previously supplied onto a copper foil (pad) located on a surface of a printed circuit substrate and a surface mounting component is positioned and mounted thereon and thereafter the resulting assembly is placed in a heating furnace (fellow furnace) to melt the solder for the connection.
(a) In this technique, a molten solder plating method, a solder paste printing method, an electrolytic solder plating method, an electroless solder plating method and the like are adopted as a method for supplying a solder for surface mounting onto the pad of the conductor circuit substrate.
However, these solder supplying methods have the following problems for conducting the surface mounting of fine pitch.
(1) In the molten solder plating method, it is difficult to control the plating thickness and a uniform plated film is not obtained.
(2) In the solder paste printing method, the limit of pitch width capable of supplying the solder is 0.3 mm and the narrowing of the pitch can not be conducted.
(3) In the electrolytic solder plating method, a lead for voltage application is required and hence the steps become complicated and the degradation of pattern wiring density results.
(4) In the electroless solder plating method, the formation of plated film is a substitute on reaction with copper and it is difficult to increase the thickness of the film.
Furthermore, there is a method in which Sn film and Pb film formed on the pad of the conductor circuit substrate through plating is alloyed to feed a solder for surface mounting as the other method of supplying the solder (JP-A-2-101190 and JP-A-4-21795). According to this method, the solder having a desired alloying ratio can easily and surely be provided on the pad of the conductor circuit substrate.
However, the latter solder-supplying method has the following problems in order to conduct the surface mounting of fine pitch:
(1) When Sn film and Pb film are formed by electrolytic plating, a lead for voltage application is required and hence the steps become complicated and the degradation of pattern wiring density results.
(2) When Sn film is formed by electroless plating, it is difficult to increase the film thickness.
(3) Since Sn film and Pb film are formed in the form of individual layers, it is difficult to conduct complete alloying in the reflow soldering.
(b) On the contrary, a super-solder technique and a self-solder QFP technique are recently proposed as a soldering technique in the surface mounting. That is, the super-solder technique is a solder producing technique in which an alloy obtained by heating reaction between Pb and Sn of organic acids is selectively precipitated onto the pad of the conductor circuit substrate. On the other hand, the self-solder QFP technique is a technique in which outer lead (pin) portions of the surface mounting component is previously subjected to a high-speed electrolytic solder plating and mounted onto the conductor circuit substrate. Certainly, it is possible to conduct the surface mounting of fine pitch onto the conductor circuit substrate according to these techniques.
However, all of these conventional techniques have a problem that the productivity is poor. That is, the super-solder technique is high in the production cost, and contains a great amount of a soldering ingredient in the reaction residue, and leaves impurities (Pb, Sn salts of organic acid) onto the plated resist to degrade the insulating property of the resist, so that there is a problem in a point that a recovery device for solid matter is required as a washing machine for production line. On the other hand, the self-solder QFP technique has a production problem in a point that the individual mounting components are required to be subjected to a plating.
As mentioned above, the conventional techniques for coping with the surface mounting of fine pitch accompanied with the rapidly progressing miniaturization of conductor circuit substrates, high-density wiring, miniaturization of surface mounting components and the like are insufficient in the thickness of the soldered layer and hardly provide the necessary electrical insulating property and are lacking in the reliability of the mounting conductor circuit substrate.